In a US election increasingly being cast as a battle between the interests of
the "haves" and the "have-nots", Michelle Obama has
emphasised her humble upbringing on the south side of Chicago.

Hardly a campaign appearance goes by the current first lady recalling the "little-bitty" apartment on the wrong side of the city where she grew up and her mother still returns when she not visiting her daughter at 1600 Pennsylvania Avenue, Washington DC.

"My father was a blue-collar worker at the city water plant. My family lived in a small apartment on the South Side of Chicago, and neither of my parents had the chance to go to college," she told supporters this week in an 1,100 word fund-raising letter.

"But let me tell you what my parents did do: They saved. They sacrificed. They poured everything they had into my brother and me.

They wanted us to have the kind of education they could only dream of."

The implicit contrast with the silver-spoon upbringing of Mitt Romney, her husband's Republican opponent come November, could not be clearer.

While Mr Romney enjoyed a cloistered start in life at the private Cranbrook prep school in Bloomfield Hills, Michigan, one of America's most affluent enclaves, Mrs Obama tells how she had to fight to carve out time for the studies that would propel her to Ivy League universities and beyond.

"Because we lived in such a little-bitty apartment it was hard to concentrate at night when everybody was awake, so often I woke up at 4:30, 5:00 in the morning just to study in quiet," she said at a campaign event last month,

"And eventually, I was accepted to Princeton University, and I went onto Harvard Law School."

In another version of the humble origins speech, Mrs Obama also makes plain that despite her personal success, her family still hasn't lost touch with its roots and the lives of ordinary voters.

"My mother still goes home there every time she's not with us. My room is the same – same bed sheets, same pictures," she said to laughter, "Everything is the same. I don't know how long she's going to keep it like that, but [more laughter] I'm okay with it, it's her house."

Mrs Obama has become a key figure in her husband's re-election campaign. Since 2008 her work supporting war veterans and campaign to tackle childhood obesity has seen her 66 per cent personal Gallup soar just as Mr Obama's – now hovering around 47 per cent – plummeted.

But citing her husband's own tough upbringing – "the son of a single mother, who struggled to put herself through school and pay the bills" – Mrs Obama said her husband's ideals of a "fair shake" for everyone, rather than Republican economic austerity, is best for America in hard times.

"Barack knows what it means when a family struggles," she concludes in her letter, "He knows what it means when someone doesn't have a chance to fulfil his or her potential. Those are the experiences that have made him the man and the president he is today."

Mrs Obama's fund-raising letter, published on the Politico website, comes as the Obama campaign tries to ramp up its efforts to raise money for an autumn campaign which polls suggest will be a close, attritional contest fought across 11 key 'swing' states.

Financial reports for the two campaigns for the month of May showed that since being confirmed as the Republican nominee, Mr Romney and his party had out-raised President Obama and the Democrats by $17m (£11m).